The NERPRC SPF program is productive and has remained pathogen-free for eight years In 1998, we incorporated 40 breeding age females and eight breeding age males, into our SPF timed-mating colony We have 75 two and three year old second generation, SPF animals that have been identified as replacement breeders and anticipate identifying an additional 50 animals born in 1998 to serve as replacement breeders Regular screening of all animals for the three target retroviruses and B Virus is the key to maintaining and deriving SPF breeding groups Established breeding groups are screened semiannually unless more frequent testing is warranted by unexpected clinical illness in the group The NERPRC SPF colony experiences high fertility and minimal fighting The pregnancy rate in 1998 was approximately 60% There are 20 stable harem groups and twelve small all female timed-mating groups New harem groups are established as space allows Small harems are recruited into timed-mating groups to maximize space The SPF breeding colony is evaluated annually and older or unproductive animals are assigned to other studies